U.S. Pat. No. 3,512,449 shows an automatic gun wherein recoil movement of the barrel causes an impactor piston to drive a bolt carrier rearwardly at an accelerated rate (greater than the barrel recoil speed). As the bolt carrier moves rearwardly the gun bolt is rotated to the unlocked position; the bolt moves rearwardly away from the barrel to enable an extracting claw on the bolt to withdraw the spent cartridge casing from the firing chamber. As the bolt carrier reaches its rearmost position an ejecting pin slides forwardly to eject the spent cartridge casing from the bolt. During the counter recoil period a compressed coil spring propels the bolt carrier forwardly; a cam mechanism associated with the bolt carrier feeds a new round of ammunition into the space vacated by the spent cartridge casing.
Automatic guns of the above type require a rapid but controlled motion of the bolt carrier in order that all of the required motions occur within the available time interval (less than 0.2 seconds in some cases). To give the bolt carrier a quick starting action in the rearward direction it has been proposed to utilize the combustion gases in the barrel as an operating force; see for example U.S. Pat. No. 2,393,627 to J. C. Garand. Such combustion gases are generated prior to the start of the barrel recoil movement; by diverting some of the hot combustion gases against the impactor piston it is possible to achieve a very high operating force during the barrel recoil movement.
In one existing arrangement the gun barrel delivers a mechanical operating force to the impactor piston via a hollow tubular gas tube that extends into the cylinder that guides the impactor piston. As the gun is fired some of the hot combustion gas is diverted from the barrel into the tubular gas tube, where it is momentarily trapped. During the recoil period the gas tube delivers a mechanical force to the impactor piston; additionally the hot gases are fully released through the tube to produce an added force on the piston.
Proper flow of combustion gases through the tube requires a minimum escape of gas across the clearance space between the tube outer surface and the cylinder inner surface. Providing this minimum escape or leakage has been a problem. A related difficulty is potential binding of the gas tube on the cylinder wall. If the tube is made to have too tight a fit in the cylinder there may be binding, with possible malfunction and/or degradation of service life. If the tube has an extremely loose fit in the cylinder the leakage flow may be too great for satisfactory impact force.
The gas tube is carried by a yoke that depends from the barrel, whereas the cylinder is mounted on the underside of the receiver. Manufacturing tolerances and slide clearances are such that it was very difficult to provide the desired fit of the gas tube in the cylinder. In some instances it was necessary to delay final machining until after the gun components had been partially assembled together. The guns produced by this procedure were costly and difficult to service because the parts were custom fit components that were not functionally interchangeable with other spare parts having the same nominal dimensions.
The present invention relates to a floating seal assembly that can be installed in the cylinder to provide a desired fit on the gas tube without producing the binding action that has sometimes occurred using standard sealing procedures. The floating seal assembly of the invention is able to move radially under the impetus of the gas tube, so that the tube can slide freely back and forth without binding on the cylinder wall. Radial movements of the seal assembly avoid the necessity for match drilling the cylinder to custom fit peculiar gas tubes or tube eccentricities. The invention reduces manufacturing costs and also increases field service capabilities.